This invention relates to semiconductor devices and, more particularly, to electrical contacts to In-based devices such as light emitting diodes (LEDs).
Alloyed metallizations, involving a dopant in a gold matrix, form low specific-contact resistance electrical contacts to Group III-V compound semiconductors. However, it is well established that gold-based metallizations interact strongly with Group III-V semiconductors. GaAs/GaAlAs and InP/InGaAsP LEDs using gold-based contacts to p-type material (i.e., "p-contacts") degrade rapidly when dark, nonradiative areas, known as dark spot defects (DSDs) eventually develop within the light-emitting region. These dark areas form because gold from the p-contact migrates into various epitaxial layers as described by A. K. Chin et al, IEEE Transactions Electron Devices, Vol. 30, p. 304 (1983). The gold migration is accelerated by elevated temperatures and high current densities required in some transmission system applications.
To improve device reliability, nongold-based p-contacts such as nonsintered Ti/Pt have been used between the semiconductor and a gold bonding layer. However, in order to obtain a low specific-contact resistance, the semiconductor wafer must first be diffused with a p-type dopant such as Zn. This diffusion step is often responsible for variations in contact resistance as described by M. Ettenberg et al, Journal of Vacuum Sciences and Technology, Vol. 19, p. 799 (1981).